Various thermal transfer recording methods have been known so far. Among these methods, dye diffusion transfer recording systems attract attention as a process that can produce a color hard copy having an image quality closest to that of silver halide photography (see, for example, “Joho Kiroku (Hard Copy) to Sono Zairyo no Shintenkai (Information Recording (Hard Copy) and New Development of Recording Materials)” published by Toray Research Center Inc., 1993, pp. 241-285; and “Printer Zairyo no Kaihatsu (Development of Printer Materials)” published by CMC Publishing Co., Ltd., 1995, p. 180). Moreover, this system has advantages over silver halide photography: it is a dry system, it enables direct visualization from digital data, it makes reproduction simple, and the like.
In this dye diffusion transfer recording system, a heat-sensitive transfer sheet (hereinafter also referred to as an ink sheet) containing dyes is superposed on a heat-sensitive transfer image-receiving sheet (hereinafter also referred to as an image-receiving sheet), and then the ink sheet is heated by a thermal head whose exothermic action is controlled by electric signals, in order to transfer the dyes contained in the ink sheet to the image-receiving sheet, thereby recording an image information. Three colors: cyan, magenta, and yellow, or four colors which consists of the three colors and black, are used for recording a color image by overlapping one color to other, thereby enabling transferring and recording a color image having continuous gradation for color densities.
It is preferable that thermal transfer layers of individual colors are formed repeatedly in a frame sequential mode on one and the same support. In addition, part of thermal transfer layers may be ink layers of heat-fusion transfer type. Furthermore, a protective layer for covering and protecting printed images through thermal transfer onto images after printing can also be formed on the same support in addition to the thermal transfer layers.
The thermal transfer layers can be provided in any manner as long as it is appropriate to the intended purpose. For instance, it is possible to provide thermal transfer layers of color hues other than general yellow, magenta, cyan or black hue.
In this dye diffusion transfer recording system, a heat-sensitive transfer sheet containing dyes is superposed on a heat-sensitive transfer image-receiving sheet (hereinafter also referred to as an image-receiving sheet), and then the ink sheet is heated by a thermal head whose exothermic action is controlled by electric signals, in order to transfer the dyes contained in the ink sheet to the image-receiving sheet, thereby recording an image information. Three colors: cyan, magenta, and yellow, are used for recording a color image by overlapping one color to other, thereby enabling transferring and recording a color image having continuous gradation for color densities.
However, utilization of the heat-sensitive transfer recording methods presented drawbacks that fusion bonding or sticking occurred between an image-receiving sheet for heat-sensitive transfer recording and a heat-sensitive transfer sheet, wrinkles developed in a heat-sensitive transfer sheet by heat or pressure applied to the sheet at the time of thermal transfer recording, and so on.
As methods for preventing image-receiving paper from suffering heat-fusion bonding on the color-material reception layer side, JP-A-60-34898 (“JP-A” means unexamined published Japanese patent application) discloses incorporation of a dye-permeable release agent into a receptive layer (image-receiving layer), and JP-A-61-258792 discloses formation of a thin layer of cured silicone resin on a dyed resin layer.
As disclosed in JP-A-2-196692 and JP-A-8-2126, there is known the method of resolving heat-fusion bonding or sticking troubles occurring between an image-receiving sheet for thermal transfer recording and a heat-sensitive transfer sheet by incorporation of a silicone resin into the heat-sensitive transfer sheet. However, there is a trade-off relation that, when a silicone resin in a higher amount is incorporated into a heat-sensitive transfer sheet, the heat-sensitive transfer sheet is more likely to develop wrinkles (ribbon wrinkles). Solution of this trade-off problem from the material point of view has not been found yet.
In addition, incorporation of a high amount of silicone resin into a cyan thermal transfer layer in particular causes another problem of degrading surface conditions of the coating layer and making it difficult for the printed image to have sufficient density.